1. Field of the Invention
This invention relates generally to a frequency down-converter and, more particularly, to a frequency down-converter for a receiver in a wireless telecommunications system, where the down-converter simultaneously down-converts multiple signal channels received by the receiver.
2. Discussion of the Related Art
Wireless telecommunications systems, particularly cellular telephone communications systems, employ strategically placed base stations having transceivers that receive and transmit signals over a particular carrier frequency band to provide wireless communications between two parties. Depending on the particular area, each base station will include a certain number of receivers that receive the signals, and provide digital signal processing so that the signal is transmitted to the desired destination. Each receiver scans the frequency band of interest until it locks onto a signal in that band. The frequency band is usually at about 800 MHz and higher with a bandwidth of 200 KHz or more. An example with a bandwidth of 75 MHz centered at about 1750 MHz will be used in this description.
A typical receiver for this application will include an antenna that receives the signals and a duplexer that limits the received signals to the desirable carrier frequency range in which the signals are transmitted. The frequency band limited signal received by the antenna is then applied to a low noise amplifier (LNA) where it is amplified to an amplitude suitable for subsequent processing. The band limited and amplified signal is then applied to a-frequency down-converter that mixes the received signal with a local oscillator (LO) signal to generate a lower intermediate frequency (IF) signal that can be readily processed by the DSP devices. A bandpass filter (BPF) is typically employed in the frequency down-converter to limit the IF signal to the particular frequency band of the received signal. The filtered IF signal is then applied to an analog-to-digital converter (ADC) that converts the IF signal to a representative digital signal that is then processed by the DSP devices. The receiver may be designed to process signals for many protocols and standards, including time division multiple access (TDMA), code division multiple access (CDMA), enhanced data rate for global evolution (EDGE), Gaussian minimal shift keying (GMSK), etc., depending on the particular application.
The known receiver described above for wireless telecommunications has only been capable of processing a single signal or a single channel at any given time. Thus, a separate receiver has been required in the base station for each separate signal being processed. Because certain base stations may have to receive and transmit many signals for multiple users simultaneously, these base stations must include enough receivers to satisfy this requirement, or service is lost. Because each receiver includes the various components that perform the signal receiving operation discussed above, high traffic base stations include a large amount of receiver hardware, and thus are costly. As cellular telephone communications increases, base stations will be required to provide more simultaneous access, or more base stations will be required, thus significantly increasing costs in these systems.
What is needed is a wireless telecommunications system receiver that operates over a relatively wide bandwidth, and is capable of processing multiple signals in different frequency bands simultaneously. To process multiple signals simultaneously, the basic components, including the LNA, the frequency down-converter and the ADC, of the receiver need to be redesigned to provide the performance specifications and requirements necessary. Compared to the single channel receiver previously mentioned, the multiple channel receiver must continuously handle a much wider range of frequencies and larger range of power levels. In addition, the multiple channel receiver must be insensitive to potential cross-channel interference. It is therefore an object of the present invention to provide such a receiver having high performance components.
In accordance with the teachings of the present invention, a frequency down-converter for a receiver in a wireless telecommunications system is disclosed. The down-converter down-converts a high frequency signal to an intermediate frequency or baseband signal. The down-converter is capable of simultaneously processing a plurality of signal channels without increased signal distortion over a relatively wide bandwidth. The frequency down-converter employs a suitable mixer, BPF and attenuator that are tuned to provide the desired frequency down-conversion and amplitude control over the desired wide bandwidth. In one embodiment, the BPF passes a 25 MHz or above bandwidth. The frequency down-converter generates the intermediate frequency signal in a single step down-conversion process, or generates the intermediate frequency signal and then a baseband signal in a two step down-conversion process.